Felix Writes: Fun With Time Travel

A recent Tweet pointed me at this funny story, which
the tweeter jokingly suggested is indicative of time travelers. Never
mind that actual time travelers would be a lot more awkward.
(Anyone cares to guess at a rational explanation for the weird couple?)
In any event, that prompted me to think about time travel again, and
all the problems associated with the concept.

Sure, at the movies it's pretty simple: the big bad cyborg goes back
in time to change history. Fireworks ensue. Things become complicated
when it turns out the trip itself was what enabled the future cyborg
to be built at all. That's called a stable time loop, and it's crazy
enough to give some people headaches. But it can be worse. Ever heard
of the grandfather paradox?
It goes like this: I went back in time to kill my grandfather... but
now I was never born in the first place... so I couldn't go back in
time to kill him... so I was born after all... so I went back in time...
That's the exact opposite of a stable time loop, and it's unclear what
would actually happen.

But that assumes changing history is possible in the first place, and
if you stop to think about it, the grandfather paradox would apply to
any such attempt. Let's take everyone's favorite example: killing
Hitler before he gets nominated Chancellor. So, I went back in time
and did it; now World War II never happened... which means I have no
reason to go back in time... so nobody killed Hitler... so WW2 did
happen after all... so I decide to go back in time...

Fun!

But even if history can be changed, there's another problem few people
grasp: how would anyone ever know it? Think about it: someone goes
back in time and kills Hitler. Now every single one of us will have
grown up in a world where there hasn't been a second world war. For
all intents and purposes, that's now THE history. How can you tell
it was made by someone from the future, and not one of the dozens of
Hitler contemporaries who actually tried to kill him back then?

Or perhaps you'd rather go with the many-worlds interpretation.
So, you went back and made the big change. Now there are two parallel
timelines: one in which the change never took place, and one where
things were different from the start. In either timeline, for those
who came after the events, history was always that way; there's no way
to tell anything "changed".

For all we know, time travelers could be hopping back and forth all
the time, tweaking events here and there. There's just no way to tell.

Luckily, this probably isn't happening. Relativity tells us it takes
literally cosmic amounts of energy to bend time and space even a tiny
bit; the scale required for time travel is just too big. But there's
another possibility: if recent results from the Large Hadron Collider
are correct, neutrinos may be traveling faster than light.
If that's true, it means they're also moving backwards in time like
the hypothetical tachyons... which means we could use them to message
our past selves! The bad news is, of course, that neutrinos move only
a tiny bit faster than light, and since they also move through space,
that means you can only send messages into the past between distant
stars. Not really useful until we have a galactic empire.

Don't despair, though! All this thinking about sending stuff into the
past obscures a perfectly feasible way to travel in time. It's called
"waiting", and while it can only take you forward, that can be
remarkably useful. Don't believe me? Watch the mindblowing Dr. Who
episode Blink... or ask any historian. They even invented
something called time capsules for this very purpose.

I took such leaps in time myself, when visiting certain neighborhoods
of Bucharest for the first time after twenty years (and the 1989
Revolution). It's a special experience that makes me think about roads
not taken, and what we can do now to influence the future. Because for
now that's the best we may hope for, and it matters, as we're going to
live in that future.

Too bad we can't have the hindsight of a time traveler. Be wise instead.